Black mark on nails is part of Shakun in Hindu astrology. It is considered bad luck to see black mark on nails. These black marks come and disappear pretty quickly.
Black mark on left hand thumb nail suggests accident and on right hand thumb nail suggests relationships problems with family members, spouse, lover or friends.
Black mark on left hand index finger nail is associated with diseases and on right hand index finger nail is associated with problems or theft during travel.
Black mark on left hand middle finger nail is associated with loss valuables and on right hand middle finger nail is associated with death of family member or friend.
Black mark on left hand ring finger nail is associated with financial loss and on right hand ring finger nail is associated with job loss or shame or scandal.
Black mark on left hand small or pinky finger nail is associated property loss or trouble in business or problems due to bad weather and on right hand pinky finger or small finger nail is associated with bad luck in money and career matters.